There are various kinds of data carriers, and there are many methods to carry out bidirectional noncontact data communication between a data carrier and a fixed data control unit that is separated from the data carrier by a given distance.
Japanese Examined Patent Publication Nos. 3-12353 and 63-25393, Japanese Unexamined Patent Publication Nos. 51-15947 and 57-32144, U.S. Pat. No. 3,964,024, etc., disclose methods for carrying out bidirectional noncontact data communication between a portable data carrier such as a card and fixed facilities.
Most of these data carriers incorporate batteries. The distance between the data carrier and the fixed facilities having a control unit for carrying out the bidirectional noncontact data communication is usually fixed. Accordingly, errors rarely occur in data write and read operations.
As communication systems employing the data carriers become widely used, a need rises to reliably transfer data even if the distance between the data carriers and the fixed facilities varies widely. When the distance varies, the voltage of a transmitted signal detected at the data carriers and fixed facilities fluctuates. Accordingly, it will be impossible to bidirectionally transmit data with no errors or malfunctions.
The conventional data carriers having no batteries are incapable of writing and reading data or transmitting data to and from fixed facilities when a distance between them varies in a range of several centimeters to several meters. In particular, the presently marketed electromagnetic coupled data carriers having no batteries are only capable of reading data that are permanently or semi-permanently recorded. Various experiments have been made to freely rewrite data in a data carrier during use. Since an EEPROM used as a data storage unit needs a large rewrite voltage and since the EEPROM has insufficient reliability as a memory, no practical data carrier that is freely erasable and programmable has been proposed.
To solve the problems of the EEPROM, experiments have been made to use an SRAM as a data storage unit of the data carriers. Several data carriers employing SRAMs have been produced. This type of data carrier, however, must incorporate a battery. The battery has a limited service life and insufficient reliability. In addition, the battery increases the weight and size of the data carrier.
Among the prior arts, the Japanese Examined Patent Publication No. 3-12353 discloses a data carrier that is capable of writing and reading data, provided that a distance between the data carrier and fixed facilities is less than several centimeters. Due to this short distance, electric power can be easily supplied from the fixed facilities to the data carrier. Generally, this type of product is distinguished from the data carrier and is called a noncontact memory card.
The electromagnetic coupled data carrier having no power source frequently suffers a loss of voltage due to insufficient power from the fixed facilities. This results in incorrect logic operations of circuits in the data carrier, and it erroneously rewrites the electrically erasable programmable nonvolatile memory incorporated in the data carrier. To prevent such an accident, a rewrite control circuit of the nonvolatile memory must not be activated unless a supplied voltage is sufficiently high to secure a correct operation.
Accordingly, a main object of the present invention is to develop an electromagnetic coupling data carrier that solves the problems of the prior art, is capable of storing and reading data with no battery, stably transmits data to and from fixed facilities with a distance between them of more than several centimeters, and has improved data storage reliability. The data carrier employs a low power consumption high performance CMOS IC having a nonvolatile memory called a MONOS memory that is erasable and writable under low power conditions.